Those skilled in the art of drilling hydrocarbon recovery wells have long recognized the benefits of downhole stabilizers placed at strategic locations within the drill string. Numerous advances have been made to the design, material construction, and operation of stabilizers which have enhanced drilling operations, and thereby lowered hydrocarbon recovery costs. While drill string stabilizers have utility in borehole operations which are not related to hydrocarbon recovery, their primary purpose relates to use in hydrocarbon recovery wells, and accordingly that use is described herein.
One significant technological feature of downhole stabilizer relates to its ability to adjust the stabilizer diameter while the stabilizer is downhole by radially moving the stabilizer blades with respect to a fixed diameter stabilizer body. While blades in a stabilizer system have historically been "changed out" at the surface to increase or decrease the stabilizer diameter, this operation is time-consuming and thus expensive. The desirable downhole adjustment feature of a stabilizer has significant benefits with respect to selectively altering the drilling trajectory, particularly for stabilizers positioned close to the drill bit. By selectively increasing or decreasing the stabilizer diameter while downhole, drilling operators are better able to accommodate oversized holes or holes very close to gage. The drill string may be more easily tripped in and tripped out of a well bore by reducing the stabilizer diameter during this phase compared to the stabilizer's maximum diameter used in drilling operations, thereby saving substantial time and drilling costs. While wireline retrievable tools may be used for adjusting the stabilizer diameter while the stabilizer is downhole, the preferred technique for adjusting stabilizer diameter utilizes operations controlled at the surface, such as mud pump activation and weight-on-bit, to regulate this change in diameter.
One type of downhole stabilizer relies on alterations in weight-on-bit to adjust the stabilizer diameter. U.S. Pat. No. 4,572,305 to Swietlik discloses a stabilizer wherein its radial diameter is controlled by regulating the magnitude of force applied to the bit through the stabilizer. By increasing or decreasing the weight-on-bit, telescoping members affect the axial length of the stabilizer which causes cam followers to move along a cam surface to radially expand or retract stabilizer fins or blades. U.S. Pat. No. 4,754,821 discloses an improvement to this adjustable downhole stabilizer, wherein a locking device is employed to lock the stabilizer diameter, so that the axial force applied to the bit may be altered without changing the stabilizer diameter. A collar is moved to compress a spring and close a valve, which isolates hydraulic lines and locks the telescoping shafts into position.
U.S. Pat. No. 4,848,490 to Anderson discloses a downhole adjustable stabilizer, wherein a mandrel telescopes within a stabilizer casing and has cam surfaces which engage radial spacers. The stabilizer diameter is controlled by adjusting the weight-on-bit, and this control is functionally independent of hydraulic forces due to the pumping of drilling mud. A mechanical detent mechanism releases the mandrel to change the stabilizer diameter only when mechanical force above a critical value is obtained. European Patent Application 90307273.4 discloses a locking device for an adjustable stabilizer. The tool actuator is moveable by a substantial change in the fluid flow rate from a locking position to an unlocking position. The effective diameter of a downhole orifice changes between the locked and unlocked positions, and consequently a position determination can be obtained by monitoring fluid pressure at the surface.
U.S. Pat. No. 4,821,817 assigned to SMF International discloses a comparatively complicated actuator which utilizes drilling mud rather than weight-on-bit to control tool actuations. Fluid flow rate is used to regulate axial movement of a piston within the stabilizer. Stabilizer blades are moved radially in response to axial movement of a piston, with diameter changes occurring as a result of finger movement along successive inclined slopes arranged over the periphery of the piston. This toggle-type movement provides an indirect determination of the stabilizer diameter, since relative movement from any one finger level to another, which alters the cross-sectional flow passage through a port and thereby changes the head pressure at the surface, is ideally detected at the surfaces. U.S. Pat. No. 4,844,178 discloses a similar technique for operating two spaced-apart stabilizers interconnected by a common shaft. U.S. Pat. No. 4,848,488 discloses two spaced-apart stabilizers, and different flow rates may be used for independently controlling each of the stabilizers. A still further improvement in this type of adjustable downhole stabilizer is disclosed in U.S. Pat. No. 4,951,760.
U.S. Pat. No. 4,491,187 to Russell discloses an adjustable stabilizer wherein the alteration of drill string pressure are utilized to move a piston. A barrel cam mechanism is used to expand or retract the stabilizer blades. Fluid pressure within the stabilizer is equalized with fluid pressure in the well bore annulus in one embodiment, and the barrel cam mechanism is pressure balanced with internal fluid pressure in another embodiment. Pumping pressure may be reduced while the stabilizer blades are maintained in their outward position.
U.S. Pat. No. 3,627,356 discloses a deflection tool for use in directional drilling of a well bore. An upper and lower housing are pivotably connected, and a lower housing is coupled to a downhole motor to rotate the drill bit. Drilling fluid drives a piston and lever mechanism in the upper housing for urging the lower housing to pivot relative to the upper housing. A retrievable limiting probe is lowered into the deflection tool via wireline for setting a plug which limits the extent of pivotable movement. The deflection tool achieves the benefits of an adjustable bent sub, and utilizes a pressure differential between the tool bore and the well annulus to cause the pivoting movement of the upper assembly relative to the lower assembly.
The prior art adjustable downhole stabilizers have significant disadvantages which have limited their acceptance in the industry. Stabilizer adjustment techniques which require a change in weight-on-bit for activation are not preferred by drilling operators, in part because an actual weight-on-bit may be difficult to control, and since operator flexibility for altering weight-on-bit without regard to stabilizers activation is desired. Some prior art adjustable downhole stabilizers do not allow the radial position of the stabilizer blades to be reliably locked in place. Currently available downhole adjustable stabilizers have a large number of moving parts which frictionally engage, thereby reducing stabilizer reliability and increasing service and repair costs due to wear on these engaging components. Prior art stabilizers which utilize a pressure balanced system have additional complexities which further detract from their reliability and increase manufacturing and service costs. Some stabilizer adjustment techniques do not provide for monitoring the actual radial position of the stabilizer blades, but rather seek to accomplish this general goal in an indirect manner which lacks high reliability.
Improved methods and apparatus are required if the significant benefits of downhole adjustable stabilizers are to be realized in field operations. The disadvantages of the prior art are overcome by the present invention, and an improved downhole adjustable stabilizer and technique for adjusting a downhole stabilizer are hereinafter disclosed.